1. Field of the Invention
The present invention relates to a hollow fiber and a method for fabricating the same, more particularly, to a hollow fiber provided with a reflecting layer with a substantially uniform film thickness along a longitudinal direction of the hollow fiber and a method for fabricating the same.
2. Related Art
As a conventional hollow waveguide, a hollow fiber with a hollow structure which comprises a quartz material has been known. The hollow fiber has been employed as an optical transmission line for a pulsed laser light having a high peak power or for a light in an infrared (IR) wavelength band of a wavelength of 2 μm or more that cannot be used for a solid (non-porous) type optical fiber comprising a quartz material as a transmission medium since an optical transmission loss is too large.
For increasing an optical transmission efficiency in such a hollow fiber, a hollow fiber in which an inside of the hollow structure is coated with a metal film is proposed. By way of example only, Satoshi Kubota et al, “Fabrication of low loss narrow silver hollow glass waveguide by silver mirror reaction method”, Laser Engineering, The Laser Society of Japan, June 1997, Vol. 25, p 438-441 discloses the silver hollow glass waveguide.
In the hollow fiber disclosed by Kubota et al, a silver liquid made by dissolving silver nitrate and a reducing liquid containing glucose as a reducing agent are simultaneously vacuumed with a vacuum pump to be mixed with each other, and the mixed liquid is introduced into a glass capillary that is a base material of the hollow fiber, so that silver particles are deposited on an inner wall to provide a silver thin film.
Further, there is another type of a conventional hollow fiber, in which a thin film of aluminum is formed on an inner wall of a glass capillary by MOCVD (Metal Organic Chemical Vapor Deposition) method with the use of DMEAA (dimethylethylaminealane) as a source. By way of example only, Yuji Matsuura et al “Aluminum-coated hollow fiber for excimer laser”, Optical alliance, Japan Industrial Publishing Co., Ltd., July 1999, p 20-22 discloses an example of this type of the conventional hollow fiber.
However, according to the aforementioned conventional hollow fibers, there is a disadvantage in that a breakdown threshold of the hollow fiber is reduced when a laser light having an extreme high peak power of the pulsed laser light, since a stress loading based on irradiation of the laser light may become considerable in accordance with a metal particle diameter composing the metal film formed on the inner wall.
As a still another conventional hollow fiber, a hollow fiber comprising a glass capillary with a hollow structure, a silver thin film coated on an inner wall of the glass capillary, and an silver iodide thin film formed at a surface of the silver thin film is proposed. By way of example only, J. Harrington, “A Review of IR Transmitting, Hollow Waveguides”, Fiber and Integrated Optics, Vol. 19, pp. 211-227 (2000) discloses an example of this type of the conventional hollow fiber.
In the hollow fiber disclosed by Harrington, a silver liquid made by dissolving silver nitrate and a reducing liquid containing glucose as a reducing agent are simultaneously vacuumed with a vacuum pump to be mixed with each other, and the mixed liquid is introduced into a glass capillary that is a base material of the hollow fiber, so that silver particles are deposited on an inner wall to provide a silver thin film, similarly to the conventional hollow fiber disclosed by Kubota et al. (Silver mirror plating method). Subsequently, a solution in which iodine is dissolved is poured into the glass capillary to change a part of the silver thin film into a silver iodide by chemical reaction.
Since the hollow fiber disclosed by Harrington has a hollow structure, this type of the hollow fiber may be used the optical transmission line for the pulsed laser light having the high peak power or for the light in the IR wavelength band of a wavelength of 2 μm or more that cannot be used for the solid type optical fiber comprising a quartz material as the transmission medium because of the optical transmission loss. In addition, since the hollow fiber disclosed by Harrington is provided with the silver thin film and a transparent layer at the inner wall of the capillary, it is possible to reduce the transmission loss of the light in an IR region.
However, in the hollow fiber disclosed by Harrington, the silver thin film has a film thickness distribution in a longitudinal direction of the hollow fiber, since the silver thin film is formed by silver mirror electroplating method. In addition, the silver thin film formed at the inner wall of the glass capillary should be a film with a thickness of several micrometers (μm), since the silver iodide thin film is formed by carrying out an iodine process on a part of the silver thin film. Therefore, a mirror surface of the inner wall of the glass capillary cannot be succeeded to a surface of the silver iodide thin film in the hollow fiber disclosed by Harrington. Further, the film thickness of the silver iodide thin film is controlled by a contact time of the silver thin film with the solution in which the iodine is dissolved, so that a reaction rate varies in accordance with a variation in an iodine concentration and variation in solution temperature of the solution in which the iodine is dissolved, it is difficult to provide the silver iodide thin film with a smooth surface and a uniform film thickness along the longitudinal direction of the hollow fiber.